U.S. patent application number 14/584134 was filed with the patent office on 2015-07-02 for acquisition, tracking, and pointing apparatus for free space optical communications with moving focal plane array.
The applicant listed for this patent is Liam Borsodi, Charles H. Chalfant, III, Michael Leary, Terry Tidwell. Invention is credited to Liam Borsodi, Charles H. Chalfant, III, Michael Leary, Terry Tidwell.
Application Number | 20150188628 14/584134 |
Document ID | / |
Family ID | 53483119 |
Filed Date | 2015-07-02 |
United States Patent
Application |
20150188628 |
Kind Code |
A1 |
Chalfant, III; Charles H. ;
et al. |
July 2, 2015 |
Acquisition, Tracking, and Pointing Apparatus for Free Space
Optical Communications with Moving Focal Plane Array
Abstract
An acquisition, pointing, and tracking (ATP) apparatus for free
space optical (FSO) communications systems incorporates a
multi-element detector array positioned at a focal plane of an
optical telescope. An optical communications element lies at the
center of the detector array. In lieu of traditional beam steering,
the apparatus performs pointing and tracking functions internally
by first calculating a position of an optical maximum on the
detector array, and then translating the detector array within the
focal plane of the telescope such that the optical communications
element lies at the optical maximum for transmitting and/or
receiving optical communications signals.
Inventors: |
Chalfant, III; Charles H.;
(Fayetteville, AR) ; Tidwell; Terry; (West Fork,
AR) ; Leary; Michael; (Farmington, AR) ;
Borsodi; Liam; (Fayetteville, AR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Chalfant, III; Charles H.
Tidwell; Terry
Leary; Michael
Borsodi; Liam |
Fayetteville
West Fork
Farmington
Fayetteville |
AR
AR
AR
AR |
US
US
US
US |
|
|
Family ID: |
53483119 |
Appl. No.: |
14/584134 |
Filed: |
December 29, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61921331 |
Dec 27, 2013 |
|
|
|
Current U.S.
Class: |
398/131 |
Current CPC
Class: |
H04B 10/112 20130101;
G02B 13/22 20130101; G02B 23/00 20130101 |
International
Class: |
H04B 10/112 20060101
H04B010/112 |
Claims
1. An acquisition, tracking, and pointing apparatus for free space
optical communications, comprising: a. a detector array comprising
a plurality of detector elements arranged in a focal plane of an
optical telescope; b. an optical communications element positioned
within the focal plane of the optical telescope and adjacent to at
least a subset of the plurality of detector elements in the
detector array; and c. at least one actuator attached to the
detector array to translate the detector array and optical
communications element within the focal plane of the optical
telescope.
2. The apparatus of claim 1, wherein the optical communications
element is a high-speed photo-detector diode.
3. The apparatus of claim 1, wherein the optical communications
element is an optical emitter diode.
4. The apparatus of claim 1, wherein the optical communications
element is an optical fiber.
5. The apparatus of claim 2, comprising a plurality of actuators
each attached to the detector array.
6. The apparatus of claim 5, wherein a first actuator of the
plurality of actuators is positioned to move the detector array in
a first direction within the focal plane of the optical telescope,
and a second actuator of the plurality of actuators is positioned
to move the detector array in a second direction within the focal
plane of the optical telescope, wherein the first direction is
perpendicular to the second direction.
7. The apparatus of claim 1, wherein the detector array comprises
an outer circumference and wherein a light signal received through
the optical telescope falls onto the detector array within the
detector array outer circumference.
8. The apparatus of claim 1, wherein the subset of detector
elements circumscribes the optical communications element.
9. The apparatus of claim 8, wherein the optical communications
element is positioned at a geometrical center of the detector
array.
10. The apparatus of claim 1, further comprising a controller
comprising an input and an output, wherein the detector array
comprises a signal output, wherein the controller input is
connected to the detector array signal output, and wherein the
controller output is in communication with the actuator to control
movement of the actuator.
11. The apparatus of claim 10, wherein the detector array signal
output comprises a plurality of element signal outputs, each output
from a detector element in the detector array.
12. The apparatus of claim 11, wherein the controller is configured
to receive the plurality of element signal outputs, calculate an
x-y translation for the detector array, and output the controller
output to the actuator proportional to the calculated x-y
translation.
13. The apparatus of claim 12, wherein the controller is configured
to calculate the x-y translation based on a centroid of the
plurality of element signal outputs.
14. The apparatus of claim 13, wherein the controller is configured
to calculate the x-y translation based on a quad-detector centroid
of the plurality of element signal inputs.
15. An optical telescope, comprising: a. A set of lenses for
receiving or transmitting or both receiving and transmitting an
free space optical signal; b. a detector array comprising a
plurality of detector elements arranged in a focal plane behind the
set of lenses wherein the focal plane is perpendicular to a central
axis of the set of lenses; c. an optical communications element
positioned within the focal plane of the set of lenses and adjacent
to at least a subset of the plurality of detector elements in the
detector array; and d. a first actuator attached to the detector
array to translate the detector array and optical communications
element within the focal plane of the optical telescope in a first
direction; e. a second actuator attached to the detector array to
translate the detector array and optical communications element
within the focal plane of the optical telescope in a second
direction perpendicular to the first direction; f. a first motor
attached to the first actuator; g. a second motor attached to the
second actuator; and h. a controller in communication with the
first motor to actuate the first motor and in communication with
the second motor to actuate the second actuator, and further in
communication with the detector array to receive a signal from at
least a subset of the plurality of detector elements of the
detector array in order to center the optical communications
element at an optical signal maximum from the set of lenses.
16. The optical telescope of claim 15, wherein the optical
communications element is circumscribed by the plurality of
detector elements of the detector array.
17. The optical telescope of claim 16, wherein a set of first-order
adjacent set of the plurality of detector elements are directly
adjacent to the optical communications element, and wherein a
second-order adjacent set of the plurality of detector elements are
directly adjacent to the first-order adjacent set of the plurality
of detector elements.
18. The optical telescope of claim 15, wherein the optical
communications element is a high-speed photo-detector diode.
19. The apparatus of claim 1, wherein the optical communications
element is an optical emitter diode.
20. The apparatus of claim 1, wherein the optical communications
element is an optical fiber.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. provisional
patent application No. 61/921,331, filed Dec. 27, 2013, for "Rapid
Free Space Optical Link Acquisition with Moving Focal Plane
Containing an Array of Detectors." Such application is incorporated
herein by reference in its entirety.
BACKGROUND
[0002] The present invention relates to the field of optical
wireless communications, and in particular to acquisition,
tracking, and pointing (ATP) systems for free space optical (FSO)
communications.
[0003] FSO systems employ light propagating in free space to
transmit data without using a connecting cable or transmission
line. An FSO system typically consists of a set of two transmitting
terminals and receiving terminals or transceiver terminals.
Electrical communication signals are converted to optical signals,
and then transmitted from the telescope of the transmitting optical
terminal. The receiving terminal receives the incoming optical
signal into a receiving telescope, which focuses the signal into an
optical focal plane for coupling into a photodetector, which then
converts the light energy back into an electrical signal.
[0004] In order for a receiving terminal to receive an optical
signal from a transmitting terminal, the terminal telescopes must
be properly aligned. ATP components provide the beam steering
necessary for optical telescopes in FSO systems. ATP components act
to steer a transmitting telescope or receiving telescope, or both,
to point in a desired direction.
[0005] Beam steering in optical systems may be accomplished by
changing the refractive index of the medium through which the beam
is transmitted, or by the use of mirrors or lenses. One existing
beam-steering solution is motorized gimbals. A gimbal is a
mechanical apparatus to allow a suspended object to rotate freely
along two simultaneous axes, within a defined angle of view. A
gimballing system used for the alignment of an optical transmitter
or receiver typically moves the entire transmitting or receiving
telescope through the required field of view. Often, the
transmitter and receiver telescopes are mechanically coupled so
that the transmitted beam is in the exact direction of an incoming
optical beam for collection by the receiving telescope, and thus
the two telescopes operate with a common gimballing system.
[0006] Gimbal-based FSO systems may be quite heavy due to the
weight of the mechanical components, motors, and servos.
Gimbal-based systems may also be bulky due to the required
mechanical components. Finally, mechanical gimballing systems may
require the use of a great deal of electrical power, far more power
than is typically consumed by the electronics associated with an
optical receiver or transmitter system.
[0007] As an alternative to gimbal-based FSO systems, U.S. Pat.
Nos. 7,224,508, 7,612,317, 7,612,329, and 8,160,452 teach beam
steering by moving an optical fiber in the x-y focal plane of the
receiver telescope, including, for example, the use of
micro-electro-mechanical systems (MEMS) components to position the
optical fiber.
BRIEF SUMMARY
[0008] The present invention is directed to an FSO system and ATP
components for an FSO system using a multi-element array of
photo-detectors positioned in the focal plane of an optical
transmitter, receiver, or transceiver telescope. As an optical
signal is received in the telescope, that signal is detected on
certain elements of the multi-element focal plane detector array.
In response, the focal plane detector array may be repositioned
within the focal plane of the telescope. In certain
implementations, a high-speed optical detector, an optical
transmitter diode, an optical fiber, or other optical
communications element may be positioned at the center of the
multi-element detector array. The detector array is manipulated
such that the light signal input maximum is aligned with the
optical communications element. In this way, the telescope may be
aligned without the use of traditional "beam steering"
techniques.
[0009] By noting the x-y position of the incoming signal on the
telescope focal plane based on the elements of the multi-element
detector array that receive that signal, the angle of a remote
incoming optical signal may be detected, and that information may
be used to control the movement of the optical communications
element to the location of the arriving remote terminal optical
signal. The invention simplifies FSO systems by eliminating the
need for beam splitters and prisms that would be required if a
multi-element array detector were employed as a wavefront detector
and implemented remotely from the transmitter. In contrast to the
present invention, this alternative approach would require two
separate optical paths and relatively complex optical component
design, and could require mirrors for 90-degree turns.
[0010] These and other features, objects and advantages of the
disclosed subject matter will become better understood from a
consideration of the following detailed description, drawings, and
claims directed to the invention. This brief summary and the
following detailed description and drawings are exemplary only, and
are intended to provide further explanation of various
implementations without limiting the scope of the invention, which
is solely as set forth in the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 illustrates a side and end view of an optical
telescope with a focal plane array receiving an optical signal at
an edge of the focal plane array.
[0012] FIG. 2 illustrates a side and end view of an optical
telescope with a focal plane array translated such that an optical
transmission element is at the optical signal maximum.
[0013] FIG. 3 illustrates three different incoming beam angles to
an optical telescope and the corresponding locations where an
optical signal strikes a focal plane array.
[0014] FIGS. 4a, 4b, and 4c each illustrate a front view of a focal
plane array showing the location of an optical fiber, an incoming
optical signal in FIG. 4b, and the position of the focal plane
array after translation with respect to the optical signal in FIG.
4c.
[0015] FIG. 5 shows a close-up view of an optical signal received
at the focal plane array, with a three-dimensional graph of the
optical signal intensity on the focal plane array in top and side
perspective view.
[0016] FIG. 6 illustrates an implementation with a separate
receiver and transmitter telescope using a focal plane array in the
focal plane of the telescope.
[0017] FIG. 7 illustrates an implementation with a single
transceiver telescope using a focal plane array in the focal plane
of the telescope.
DETAILED DESCRIPTION
[0018] With reference to FIGS. 1 through 7, an implementation of
the present invention may now be described. A laser communications
transceiver incorporating the rapid acquisition, tracking and
pointing (ATP) system is described herein, but the invention is not
so limited, and in fact may be put to other applications where the
ATP function is desired. The optical system of FIGS. 1 through 7
consists of beam expansion optics and focusing optical design that
provides angle of arrival direction measurement and controlled
motion in the focal plane that provide immediate alignment of the
incoming optical beam. Additionally, for a transmitter telescope,
it provides controlled motion in the focal plane that aligns
precisely the outgoing transmitted beam in the direction of an
incoming optical beam from a remote optical terminal.
[0019] The focal plane contains an array of photo-detectors of
precise size to provide a correspondingly precise measurement of
the position of a focused optical beam spot that strikes the
detectors, yielding various optical signal photocurrent amplitudes
in the vicinity of the beam's focused spot. Each detector array
element that receives its relative part of the beam spot's energy
provides positional information to the focal plane's translational
system. This information from each element of the multi-element
detector array is transferred to the control system, which uses the
information to calculate a maximum beam spot. The controller moves
the optical communications element at the center of the
multi-element detector array to the maximum beam spot in the focal
plane, thereby aligning the optical communications element at the
maximum optical intensity location, and therefore synchronizing the
FSO link.
[0020] FIG. 1 illustrates the operational design of an
implementation of the free space optical communications transceiver
telescope and focal plane components described in this invention
with a high speed photo-receiver 5 as the optical communications
element. A side view is shown to the left of FIG. 1, with an end
view on the right. The telecentric telescope contains five optical
elements 1 and its wide angle input lens shows an entering beam 2
from an angle 3 that is focused onto the focal plane at a spot 4,
illuminating several of the detectors in the focal plane array
(FPA) 10 in the focal plane near the bottom edge of the array.
These detectors send signals to the control electronics identifying
the optical spot's location, which in turn commands the movement of
the focal plane assembly to align the high speed photo-receiver 5
at the center of the FPA with the exact position of the focused
optical spot 4.
[0021] FIG. 2 shows the FPA focal plane moved to its new position
so that the high speed photo-receiver has been moved and is located
at the optical beam spot location. In FIG. 2, the focal plane
assembly is moved down to the position aligning the center of FPA
10, at the high speed photo-receiver 5, with the arriving optical
beam spot. FPA 10 has been moved by the positioning motors 7 and 9
and actuators in the Y-direction 6 and in the X-direction 8 to
align the photo-receiver 5 to exactly overlap the focused spot 4;
it converts the optical signal to an electrical signal connected by
wire to the motion control electronics.
[0022] The connecting wires to photo-receiver 5 can in certain
implementations be replaced with an optical fiber matched for
maximum optical power coupling and be used for both receiving and
transmitting optical signals from remotely located electronics.
This requires that a polished optical fiber flat surface for input
and output be placed and aligned precisely within the focal plane
at the center of the FPA 10.
[0023] In certain implementations, for use in an FSO transmitter
telescope, the diode in the center of the FPA 10 is replaced by an
optical emitter such as a laser diode that transmits from the
center of FPA 10 and its focal plane of an identical telecentric
telescope. The FPA 10 detectors in this implementation function by
identifying the direction and location of a remote FSO terminal's
arriving signal beam using the same technique as described above
with respect to the high speed photo-detector 5. The location of
the focused spot arriving from a remote FSO terminal would identify
its direction of arrival via its beam spot X-Y position on the FPA
10, and command the FPA assembly to move to the X-Y position for
transmission in the direction of the arriving beam from the remote
FSO terminal.
[0024] The telecentric optical design is illustrated in FIG. 3 by
viewing it from the side for an example for various angles landing
in the focal plane along the Y-axis. The simple five-lens system 1
converts the beam arrival angles into a corresponding optical spot
in the X-Y focal plane 4. In this illustration, only the Y-axis is
shown. Three different input beam angles are depicted (2a, 2b, 2c)
arriving from the left and transformed to an X-Y location in the
focal plane on the right side where the beam spots are focused 4.
The optical design is spherical providing linearity in the X-Y
plane.
[0025] FIG. 4 depicts an end view of the focal plane with the FPA
10 and its centered high speed photo-detector 5. The first position
(4a) shows the original centered focal plane assembly position
without any beam illumination; the second shows the same position
with a beam spot 4 landing in the upper left corner onto the nearby
FPA detectors. The center of the spot will provide a strong
photocurrent to the detectors near the center of the beam, while
the detectors further from the center radially will record lower
photocurrents. The 3D beam spot is depicted in the lower right
corner of FIG. 4 illustrating the center of the spot with the
strongest optical signal and the radial reduction in optical power
that is measured by the group of detectors. All of the detectors
receiving light energy provide an electrical signal to the control
electronics, which employs a relative location algorithm to
calculate the distance and direction from the centered high speed
photo-detector receiver 10 (or optical fiber).
[0026] FIG. 5 shows a close up illustration of the FPA detectors in
the vicinity of the high speed photodetector 5 after having been
moved to the optical beam spots position. The photographs on the
right side show a beam profile measurement of an optical spot.
[0027] After the movement of the FPA assembly 10 to this receiving
location, a tracking algorithm is employed that sustains the
optimum alignment for the highest optical signal arriving to the
photo-detector. The algorithm in certain implementations uses
quad-detector centroiding. The position of the photo-receiver is
actively and rapidly updated and provides tracking of remote moving
FSO terminals. The adjacent four detectors are used for
quad-detector signal balancing centroiding. Second-order signals
from detectors that are the next neighbors to the four adjacent
detectors may also be used in the algorithm. The short term average
optical power received at the high speed photo-detector 5 is used
in the control calculations.
[0028] For an FSO transmitter telescope with an LED or laser diode
as an optical communications element, with the beam spot tracking
of a beam arriving from a remote FSO terminal, the alignment with
the remote terminal can use the same quad-detector signal balancing
as the receiver to maintain the beam alignment with the remote FSO
terminal for link synchronization.
[0029] FIG. 6 illustrates an optical telescope system that may
utilize the ATP components described above with a separate
transmitter and receiver. Two sets of lenses 1 transmit beam 2 into
and out of the device. Two FPA detectors 10 are used, with
connecting components 11 to transmit the signals received and to be
sent.
[0030] Illustrated in FIG. 7, using an optical fiber at the
location of the photo-detectors or transmitters, both transmit and
receive signals can use the same centered fiber within the same
telescope. This implementation requires optical isolation
techniques applicable to FSO systems. A transmission optical fiber
12 is connected to a fiber optic coupler/splitter 13 to separate
(and combine) the two separate optical paths from receiver 14 and
transmitter 15. This system is significantly more complicated and
includes more fiber optic components than the implementation of
FIG. 6.
[0031] The FPA detector elements rapidly determine the incoming
angle of arrival and therefore the focal plane position of the beam
spot then send this information to the ATP control loop to move the
receiver (or fiber) to the incoming angle of the beam. The high
sensitivity detectors measure the optical strength at each of the
different detectors and send this information matrix to the control
algorithm that determines the spot's exact location. To first
order, four of the detectors provide the largest signals and
provide position (angular) information. The surrounding detectors
will provide the 2nd and 3rd order accuracies that provide the most
precise information on the location of the spot, thereby sending
this information through the position (angular) detection signal
processing and algorithms.
[0032] During an acquisition scan for a terminal to terminal FSO
link, contact is made with the array at first pass in the scan. The
beam spot is measured in the matrix position, and with multiple
detectors illuminated, and with their respective positions known,
this single pass measurement is processed and the focal plane array
is moved over so that the centered device (or fiber) is aligned
with the beam spot. Each element's position, and its measured
optical power level, can be collectively integrated algorithmically
and precisely determine the position of the illuminating spot.
However, the measurement is immediate and simultaneous, so the
signal to the motion control system for the focal plane translation
stages is immediate, thereby providing an automatic alignment at
high speed.
[0033] The pattern of signals incident upon the elements that make
up multi-element FPA 10 may be represented as a two-dimensional
array. The array provides the location on the focal plane where the
most optical power is incident. The coordinates of this location
can be represented in a 2-dimensional vector R.
[0034] The FPA component is movable about the defined XY-plane,
with the defined origin being at the center of the optical fiber or
diode (0, 0). The optical power measurements seen by the detectors
in the vicinity of the focused optical spot are stored in an
n.times.n matrix, which we will call P, with each array element
represented as P.sub.ij. For the "missing" elements, that is, those
elements in P that correspond to locations where the fiber is, the
value is stored as (0, 0).
[0035] To find the horizontal component of R, we create a vector,
Ph, of the values of the sums of the sensor values in each column
of P.
Ph j = .SIGMA. l = 1 m P ij ##EQU00001##
[0036] Ph' is created from Ph by the following normalization
process.
Ph j ' = { Ph j / j = 1 n Ph j , when j = 1 n Ph j .noteq. 0 Ph j ,
when j = 1 n Ph j = 0 ##EQU00002##
[0037] The vertical coordinate of each row of the sensor array is
stored in a vector d.sub.row. The dot product of Ph' and d.sub.row
yields the horizontal component of R, which we denote as
R.sub.x.
R x = Ph ' d row . ##EQU00003##
[0038] The vertical component is found in an analogous manner. We
create a vector, Pv, of the values of the sums of the sensor values
in each row of P.
Pv i = j = 1 n P ij ##EQU00004##
[0039] Pv' is created from Pv by the following normalization
process.
Pv i ' = { Pv i / i = 1 n Pv i , when i = 1 n Pv i .noteq. 0 Pv i ,
when i = 1 n Pv i = 0 ##EQU00005##
[0040] The horizontal coordinate of each column of the sensor array
is stored in a vector d.sub.col. The dot product of Ph' and
d.sub.col yields the vertical component of R, which we denote as
R.sub.y.
R y = pv ' d col ##EQU00006##
[0041] We now have the vector, R, which points to the location,
relative to the FPA center with the optical fiber, transmitter
diode, or receiver diode, where the peak optical power is
found.
R = [ Rx , Ry ] ##EQU00007##
[0042] The control system of the free space optical communications
system is then commanded to move to the location indicated by R,
which places the peak of the optical power in the center of the FPA
10.
[0043] Since the above method allows for the detection of the peak
power location any time observable optical power is seen anywhere
on the sensor array, the acquisition process becomes simple. One or
both terminals begin a search by moving the stage in a scanning
pattern. Both terminals monitor their sensor arrays, and, if at any
time any of the sensors report a value greater than a minimum
threshold power level, the peak power location is determined by the
previously explained method. The terminal then translates the fiber
(or active transmitter or receiver) to the location indicated by
the vector R. At this point, the beam is pointed directly at the
other terminal. Since it is pointed directly at the other terminal,
it will be seen by that terminal's sensor array, and it will move
its fiber (or diode) to the location of the peak optical power. At
this point, both terminals are aimed at each other, and the
acquisition process is complete. Having acquired the location of
the other terminal, each terminal tracks the other by utilizing
quad-detector centroiding algorithms and by monitoring the power
received into the optical fiber or by the high speed detector
diode.
[0044] It may be understood that this simplified, single-step
alignment of certain implementations of the present invention does
not require the conventional internal "beam steering" techniques of
many optical telescope systems; instead, it moves the
communications detector to the location of the beam spot, thereby
requiring smaller mass movement and high precision. Additionally,
the high-speed photo detector at the center of the focal plane
array can, in certain implementations, be an optical fiber that
precisely matches the optical design of the telescope for maximum
optical power coupling efficiency; this optical fiber is connected
to the receiver and/or the optical transmitter devices remotely
inside the electronics systems of the FSO device.
[0045] The present invention in various implementations may be used
for purposes such as beam stabilization during FSO communications.
The fast-moving actuators that control position of the focal plane
array and optical communications element may compensate for vehicle
movements and vibrations when the associated optical telescope is
mounted on a vehicle. In one implementation, accelerometers send
the frequencies and directions of the vibrations of the vehicle to
the focal plane array motion controller. In this way, the response
and compensation to vibration is controlled directly in the X-Y
plane of the focal plane array.
[0046] The present invention has been described with reference to
the foregoing specific implementations. These implementations are
intended to be exemplary only, and not limiting to the full scope
of the present invention. Many variations and modifications are
possible in view of the above teachings. The invention is limited
only as set forth in the appended claims. All references cited
herein are hereby incorporated by reference to the extent not
inconsistent with the disclosure herein. Unless explicitly stated
otherwise, flows depicted herein do not require the particular
order shown, or sequential order, to achieve desirable results. In
addition, other steps may be provided, or steps may be eliminated,
from the described flows, and other components may be added to, or
removed from, the described systems. Accordingly, other
implementations are within the scope of the following claims. Any
disclosure of a range is intended to include a disclosure of all
ranges within that range and all individual values within that
range.
* * * * *